1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to high-voltage current transformers.
2. Description of the Prior Art
Instrument transformers are used to monitor voltages and currents existing in power transmission and distribution systems. Current-type instrument transformers, or current transformers, usually include a magnetic core structure having a winding disposed thereon. The magnetic core and winding assembly are suitably placed in the magnetic field created by the current to be measured. A change in the measured current produces a change in the surrounding flux; thus the voltage induced in the winding of the current transformer is changed.
Current transformers are used to provide a variety of functions, including metering and relaying. The output from the current transformer must be sufficiently representative of the measured current in order to provide the proper metering or relaying action.
The current being measured may be interrupted at any point during the polarity cycle; therefore, the possibility of creating a residual flux in the magnetic core of the current transformer is great. When using conventional core materials and construction arrangements, the residual flux may be as high as 90% of the peak flux during normal operation.
Residual flux may present errors or incorrect control of the components and circuits connected to the current transformer. When the current being measured begins to flow after a period during which the current did not flow, such as after a fault, the flux provided by the flowing current is added vectorially to the residual flux which could result in an incorrect current transformer output.
The residual flux is reduced to a nominal value after several cycles of alternating flux have been induced in the magnetic core. With the advent of metering and relaying components which respond quickly and the desirability of using these components in the first few cycles following a fault, the instantaneous response of current transformers is becoming more important.
One conventional method of reducing the residual flux in current transformers in order to provide instantaneous action utilizes air gaps in the magnetic core of such devices, as shown in U.S. Pat. No. 3,775,722 which is assigned to the assignee of the present application. As described therein, the magnetic core includes a relatively small gap which is filled with solid insulating material to maintain the spacing between adjacent layers of core laminations.
Unfortunately, reducing residual flux by constructing an air gap in the magnetic core of the current transformer increases the field intensity required to produce a given amount of flux density. To keep the flux density and field intensity relationships of the magnetic core within usable limits, the effective air gap must be kept as small as possible while still providing a significant reduction in the maximum residual flux. Although satisfactory for certain current transformer applications, such as bushing-type current transformers, the use of gapped magnetic cores in other types of current transformers, such as extra high voltage wound-type current transformers, has proved to be uneconomical and unreliable due to difficulty in maintaining gap tolerances.
Thus, it is desirable to provide a current transformer which exhibits relatively low residual flux components. It is also desirable to provide a current transformer which includes economical means for reducing residual flux components. Finally, it is desirable to provide a current transformer having reduced residual flux components in which closer control of the magnetizing characteristics is obtained compared with gapped magnetic core constructions.
Since the permeability of the magnetic cores commonly used in current transformers and, therefore, the magnetizing current in the primary winding varies with the load, the magnetizing current increases less rapidly than the voltage across the primary winding, thereby causing the ratio between the primary and secondary currents to also vary with the load. Many different arrangements have been utilized to maintain a constant ratio between the primary and secondary currents over the load range of the current transformer and for correcting phase angle differences due to varying loads.
U.S. Pat. No. 3,532,964 disclosed a current transformer construction wherein impedance elements are connected across the secondary winding to correct phase angle errors due to varying loads. Similarly, non-linear impedance elements, such as a non-linear resistor in U.S. Pat. No. 2,129,524 and a non-linear magnetic core reactor in U.S. Pat. No. 1,863,936, have been connected in parallel with the secondary winding such that the vector sum of the exciting current through the impedance element plus the exciting current through the magnetic core of the current transformer will increase linearly with the flux in the magnetic core and thereby maintain a constant ratio between the primary and secondary currents.
Although such configurations satisfactorily correct ratio and phase angle errors in current transformers, they do not significantly reduce residual flux components to levels necessary for instantaneous relaying applications. As is well known to those skilled in the art, the introduction of an impedance into the equivalent circuit of a current transformer, such as by the use of an air gap in the magnetic core, will shift the magnetization curve of the current transformer such that it intercepts the zero field intensity ordinate at a lower point than without the additional impedance. The amount of shift of the magnetization curve is proportional to the exciting current, with higher exciting currents causing greater amounts of shift and, accordingly, less residual flux components.
However, the use of additional impedances to correct ratio and phase angle errors, as shown in U.S. Pat. No. 2,129,524 and No. 1,863,936, requires substantial impedance values which have the opposite effect insofar as reducing residual flux components since higher impedances result in smaller total reactor currents compared to gapped magnetic core devices and, therefore, cause less of a reduction of a residual flux in the magnetic core of the current transformer.
What is needed is a substantially low impedance which will create a higher total reactor current for a given amount of magnetizing flux and will thereby significantly reduce residual flux components to levels required for instantaneous relaying applications.